Temperature-stable dielectric resonator filters for stripline

ABSTRACT

There is disclosed a band-reject dielectric resonator filter for stripline, the resonators being disposed over the principal conductor and separated therefrom by a dielectric spacer and offset from the symmetry position over the stripline conductor. The offset is selected so that the magnetic field lines of the stripline pass through the planar, parallel surfaces of the resonator to a maximum degree. Large scale fabrication of the stripline is facilitated, while accurate normal positioning of the resonator is assured; and resonator to stripline coupling is relatively insensitive to small variations in the lateral position. Further, the principal conductor has no substantial degradation of the resonator Q. Strong coupling has been demonstrated with the use of low dielectric constant materials that are readily-temperature compensated. Also, disclosed are a plurality of such resonators coupled together to create specially-shaped reject bands or pass bands.

United States Patent [191 Linn et al.

[ Oct. 8, 1974 TEMPERATURE-STABLE DIELECTRIC RESONATOR FILTERS FORSTRIPLINE' Inventors: Donald Floyd Linn, Kempton; James Kevin Plourde,Allentown, both of Pa.

Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

Filed: Nov. 8, 1973 Appl. No.: 413,907

Assignee:

US. Cl. 333/73 S, 333/82 B, 333/82 BT, 333/84 R, 333/84 M Field ofSearch..... 333/73 S, 83 R, 82 R82 R, 333/83 T, 84 M, 84 R, 97 R, 73 R[56] References Cited UNITED STATES PATENTS 3,713,051 1/1973 Kell 333/73R int. Cl H01p l/20,l-l01p s/os, l-lOlp 7/00.

P imr rxfitqmirrrfle s .W.-. ,L r! fi9 Assistant Examiner-MarvinNussbaum J Attorney, Agent, or Firm-Wilford L. Wisner ABSTRACT There isdisclosed a band-reject dielectric resonator filter for stripline, theresonators being disposed over i the principal conductor and separatedtherefrom by a dielectric spacer and offset from the symmetry positionover the stripline conductor. The offset is selected so that themagnetic field lines of the stripline pass through the planar, parallelsurfaces of the resonator 1 to a maximum degree. Large scale fabricationof the stripline is facilitated, while accurate normal position- 1 ingof the resonator is assured; and resonator to stripline coupling isrelatively insensitive tosmall varia- Q tions in the lateral position.Further, the principal conf ductor has no substantial degradation of theresonator 1 Q. Strong coupling has been demonstrated with the use of lowdielectric constant materials that are readily-temperature compensated.Also, disclosed are 7 a plurality of such resonators coupled together tocre- I ate specially-shaped reject bands or pass bands.

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SUBSTRATE H4 RESONATOR STRIPLINE n5 TEMPERATURE-STABLE DIELECTRICRESONATOR FILTERS FOR STRIPLINE BACKGROUND OF THE INVENTION Thisinvention relates to filters for stripline in which dielectricresonators are used.

A variety of techniques has been employed for filter ing in striplinecircuits. Some of these, such as specific configurations of the copperof the principal conductor of the stripline, are limited in performanceby the dissipation losses of the stripline resonators.

In the past several years a variety of discoveries relating to the useof dielectric resonator type filters for stripline have been made. Ithas been found generally desirable to use dielectric resonators in whichthe dielectric constant of the material, e.g., titanium dioxide (TiO isat least about 100.

Unfortunately, such dielectric resonators experience an undue variationof their filtering properties with temperature change. In response tothe need for a solution to the problem, relatively temperaturestablematerials were discovered. These materials, such as ceramic bariumtitanite (Ba2TI9 20) r a composite resonator structure including lithiumtantalate (LiTaO have been used.

As pointed out in the July 1971 article by Tor Dag Iveland, DielectricResonator Filters for Application in Microwave Integrated Circuits, IEEETransactions on Microwave Theory and Techniques, Vol. MIT-19 page 643 to644, the introduction of micro-integrated circuits has emphasized theneed for high quality resonator elements for stripline with very smallsize so that the ground plane, or housing, configuration does not haveto be unduly bulky for the integrated circuit geometry. Quoting thearticle, the coupling mechanism is essentially based on the evanescentguide technique, but apart from I-Iarrisons design, the couplingstructure is kept in the plane of the substrate, containing both thefilter and the connected circuits. The most recent description ofHarrisons design is found in the article by A. Fox, Temperature-StableLow-Loss Microwave Filters Using Dielectric Resonators," ElectronicsLetters (GB), Vol. 8, page 582, Nov. 16, 1972. In that configuration thegeometry is uniquely a cylindrical geometry. It is used primarily forcoupling from coaxial cable to waveguide; and the only stripline portionthereof is in the coupling from the coaxial cable to the waveguide. Thedielectric resonator is used as a part of the coupling arrangement.

It is therefore seen that, except for specialized applications, alldielectric resonator filters for stripline have been most convenientlylocated on the stripline substrate.

One problem associated with this configuration is that the degree ofcoupling is extremely sensitive to small variations in lateralpositioning of the resonator. This fact makes it extremely difficult toplace the resonator on the stripline as a part of the ordinaryintegrated circuit production technique. Moreover, integrated circuittechniques are impaired because of the substantial enlargement of theground plane structure that is needed in the vicinity of the dielectricresonator.

SUMMARY OF THE INVENTION According to our invention, the problems ofaccuracy of placement of a dielectric resonator with respect to astripline are solved -in a manner consistent with good temperaturestability and overall compactness by positioning the dielectricresonator over the principal conductor of the stripline and providingprecision spacing from the principal conductor in the normal directionby an accurately machined dielectric spacer.

It is one subsidiary feature of this novel configuration that an offsetfrom the symmetry positioning over the stripline conductor is selectedso that its magnetic field lines pass through the planar, parallelsurfaces of the pillbox-shaped resonator to a maximum degree. Since themaximum coupling condition is characterized by a broad, flat maximumwith respect to lateral positioning from the principal conductor, thatcoupling value is relatively insensitive to small variations in lateralposition.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of ourinvention will become apparent from the following detailed descriptiontaken together with the drawing,'in which:

FIG. 1 is a pictorial cross-sectional view of the typical prior artarrangement of dielectric resonators with re spect to striplines;

FIG. 2 is a pictorial cross-sectional view of a striplin with groundplane and with a dielectric resonator arranged with respecttheretoaccording to our invention;

FIG. 3 shows curves useful in explaining the characteristics of ourinvention;

FIG. 4 shows a modification of the embodiment of FIG. 2, using coupledmultiple dielectric resonators shown in pictorial crosssectional viewalong the stripline;

FIG. 5 shows a plan view of the embodiment of FIG.

FIGS. 6, 7 and 8 show the filter characteristic for the embodiment ofFIGS. 4 and 5;

FIGS. 9A and 9B show pictorial plan and elevation views of a pass-bandfilter employing the invention;

FIGS. 10A and 108 show a modification of the embodiment of FIGS. 9A and9B;

FIGS. 1 1 and 13 show curves useful in explaining the embodiment ofFIGS. 9A and 9B; and

FIG. 12 is a partial showing of FIG. 9A and setting forth parametersused in the curves of FIG. 11.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS In the prior art embodimentshown in FIG. 1 which has been previously characterized as the typicalprior art, the dielectric resonator 11 is coupled to the strip line 13by positioning it on the substrate 12 next to the principal conductor14. The stripline 13 also includes the dielectric layers 15 and 16,which may be air where desired, and the ground plane 17. With the use ofrelatively low dielectric constant resonators, e.g., e- 50, thestripline housing width becomes excessive and allows spurious waveguidetype modes to propagate, resuiting in undesirable spurious filterresponses.

A further drawback found with the arrangement of FIG. 1 is that thedielectric resonator 11 is not located at the maximum-coupling positionwith respect to the principal conductor; and the degree of coupling isextraordinarily sensitive to very minute variations in the lateralcenter-to-center spacing d between the principal conductors 14 and theresonator 1 1. In contrast, an arrangement of a dielectric resonator 21with respect to a stripline 23 as shown in FIG. 2 overcomes theforegoing problems by disposing the dielectric resonator 21 over theprincipal conductor 24 of the stripline so that in addition to a lateralcenter-to-center space d the resonator positioning is now characterizedby a normal center-to-center spacing h. This normal positioning isdetermined by a dielectric spacer 25, which may be accurately machinedin a separate operation from that of positioning the resonator 21 overthe substrate 22.

For completeness of illustration the stripline 23 is shown with itssubstrate 22 and the additional layer of dielectric material 26thereunder for structural rigidity. Dielectric layer 26 may be omittedwhere desired. Typically, the ground plane 27 in the vicinity of theresonator 21 encloses the resonator within a metal housing, similar to asection of waveguide, through which a tuning screw 28 is readilyinserted.

It is particularly important in the embodiment of our invention shown inFIG. 2 that the spacing parameter d is selected for maximum coupling. Inparticular, the coupling is absolutely a maximum with respect to thelateral offset d and is relatively insensitive to small variations in dbecause the maximum is a broad flat maximum. Illustratively, theincreased top ground plane clearance shown in FIG. 2 is provided in thevicinity of dielectric resonators only so that spurious housing reso- Inances are eliminated. In other words, the same characteristic impedanceas in the preceding and following stripline portions should bemaintained throughout the resonator region, for frequencies outside thereject band of the filter.

Illustratively, the width of the principal conductor 24 will be alteredin the vicinity of resonators such as reso-' nator 21 in order toprovide an impedance match along the line at these points and thus avoidspurious reflections along the stripline.

We have found that relatively low dielectric constant resonators(e'-"38) can be used in this configuration. One exemplary embodimentemployed barium titanate (Baal 1902 It is expected that theconfiguration of FIG. 2 will be found very useful in radio relayapplications and similar applications, where its reduced size and weightas compared to alternative filters will be advantageous.

The band reject filter comprising dielectric resonator 21 utilizeseither a composition material of lithium tantalate and titanium dioxideor, alternatively, uses barium titanate sa mo The use of a lithiumtantalate and titanium dioxide composite is disclosed in some detail inthe copending patent application of one of us, J. K. Plourde, Ser. No.317,385 filed Dec. 22, 1972, as-

signed to the assignee hereof. The use of Ba Ti O is disclosed in thecopending patent application of H. M. OBryan-J.. K. Plourde-J. Thomson,Jr. Ser. No. 394,187 filed Sept. 4, 1973 and assigned to the assigneehereof.

In the operation of the embodiments of FIG. 2 between 3.7 and 4.2 GHZ anout-of-band return loss greater than 25 dB was obtained; and spurious orwaveguide type mode resonances in the stripline housing or ground plane27 were eliminated.

Although higher order modes or multiple resonator modes may possibly beused to advantage for particular filter applications, only the lowestorder cylindrical mode, the TE, 5 was considered in our earlyembodimerits.

In FIG. 3, there are shown curves 31, 32 and 33 of the resonatorexternal Q with respect to the lateral offset spacing d for variousvalues of the normal spacing h. The desirable value of d is in everycase at the low part of the curve which corresponds to maximum couplingbetween the principal conductor 24 and the resonator 21. It will benoted that the resonator and therefore the coupling is insensitive tosmall variations in d about the preferred value. Because of this fact,the tolerance in d can be relaxed; and only the value h need beaccurately controlled. The precise control in the value of h is easilyobtained by positioning the resonator 21 on a dielectric spacer 25, suchas a Rexolite disc. The height of the resonator 21 was illustratively0.175 inch and its radius in the horizontal plane was 0.312 inch. Itwill be noted that, whereas in the prior art configuration the spacingdis significantly greater than the resonator radius, in ourconfiguration the spacing at can be significantly less than theresonator radius with a corresponding reduction of the required lateraldimensions of the housing 27 and a reduction in spurious resonances.

For a multiple-resonator bandstop filter as shown in FIG. 4, the spacingbetween dielectric resonators 41,

51 and 61 is an odd multiple of a quarter wavelength. The minimumallowable resonator spacing will be the spacing for which the spuriouscoupling between resonators is held to an acceptable level and isdetermined by the excitation level of the higher order modes.Advantageously, a housing possessing small, crosssectional dimensions,according to our invention, will permit the resonators to be closelyspaced. Illustratively, each resonator 41, 51 and 61 is provided withits own tuning screw 48, S8, and 68. The entire assembly 43 is shownwith the coaxial couplings 69 and 70. In FIG. 5 a plane view in sectionshowing principally the configuration of the principal conductor 44illustrates where the impedance-matching changes in width of theprincipal conductor occur with respect to the lateral dimensions of theresonators 41, 51 and 61. Specifically, these changes in width arevertically aligned with the resonators. The tuning screws 48, 58 and 68allow plus frequency tuning when the screw is a conductor or minusfrequency tuning when the screw is a dielectric. In other respects thedesign and operation of the filters of FIG. 4 and 5 follow knownprinciples for dielectric resonators used with stripline.

Some typical filter characteristics are shown in FIGS. 6:8; The curves71, 81 generally show the return loss S i versus frequency; the curves72, 82 show transmission 21 versus frequency.

Characteristics for a filter utilizing Ba Ti O resonators arerepresented in FIG. 6. The unloaded resonator Q, Q can be calculatedgiving an approximate value of 6,300.

The above Q, value compares well with the undegraded resonator Q of6,780 verifying that the Q degradation is not significant with thisfilter configuration. The out-of-band return loss over 3.7 to 4.2 GHZ is26dB. This parameter is determined by the uniformity of the striplineimpedance throughout the filter rather than upon the resonatorproperties themselves. The peak insertion loss is approximately given bythe following equation:

Equation 1 gives 104 dB whereas the measured value is 84 dB. M's.

Curves 81 and 82, l ul and IS versus frequency for a filter utilizingLiTaO /T i0; composite resonators are given in FIG. 7. Q 2,800, showingsome Q, degradation from the undegraded resonator Q of 3,820. Thecoupling Q QC, omens) /Qo)" -v the Q associated with the dissipationlosses external to the resonators is 10,463. The out-of-band return lossis measured at 28 dB. Equation 2 yields a peak insertion loss of 91 dBwhereas the measured value is 64 dB. The outof-band insertion loss forthe BazTigozo and Li- TaO /TiO filters is 0.10 dB and 0.175 dB,respectively. This parameter is essentially independent of thedielectric resonator properties and depends upon the quality of thetransmission line used in the filter.

In FIG. 8 the curve 91 shows the transmission characteristic of themultiple resonator filter versus frequency at 40 Fahrenheit. The sametransmission versus frequency characteristic is shown by the displacedcurve 92 for a temperature of 141 Fahrenheit. The shift thencharacteristically was 2.625 MHz. This value is less than the 3.17 MHzvalue shift predicted from the temperature coefficient of an isolatedresonator because of a small compensating effect related to the use ofan alumina substrate 42 and the. metal filter housing 47, for the caseof use of barium titanate in the resonators 41, 51 and 61. Thetemperature coefficient of frequency is positive for barium titanate andis negative for alumina and the housing effect. The resonators used inthis filter possessed an average temperature coefficient of frequency,1, 14.3 ppmC. Similar materials for dielectric resonators can be made toyield temperature coefficients as low as 7; 2.5 ppmC. A practicaltolerance range would be r 0 i ppm/"C with dielectric resonatortolerances dominating. Such filters are essentially temperaturecompensated. It is found that the abovedescribed filter characteristicsare far superior to those of a typical copper comb-type filter used withstripline.

While it is well known for other types of transmission media and otherresonator-filters that a band-pass filter can be constructed bymodifying a bandstop filter, it is instructive to consider how thoseprinciples are applicable to the present invention. A band-pass filteris formed by positioning one or more resonators in a section ofwaveguide that is beyond cut-off at the frequen: cies of interest, inthe absence of the resonators.

In the illustrative band-pass filter of FIG. 9A, the dielectricresonators 103, which are like resonator 21 of FIG. 2 or respectivelylike resonators 41, 51 and 61 of FIGS. 4 and 5, are spaced apart inhousing 101 above stripline 102.

The housing 1011s a section of waveguide, illustratively rectangular,which is beyond cut-off; that is, it will not propagate the frequenciesof the intended passband in the absence of resonators 103. The intendedpass-band frequencies lie in a band which is centered at the resonantfrequencies of the resonators 103. Electromagnetic energy is coupledthrough the structure from left to right in the drawing. At otherfrequencies, outside of the pass-band, very little energy is propagatedthrough the structure.

While both direct-coupled and quarterwave-coupled band-pass filters willbe described, FIGS. 9A and 9B show the direct-coupled band-pass filter.The input and output sections of stripline 102 are coupled to the endresonators 103 and the inner ones of the resonators 103 directly coupleenergy to adjacent resonators. The resonators couplings and hence thefilter characteristics are determined by the inter-resonator spacingsbetween the inner resonators as well as the coupling between the endresonators and the input and output striplines.

As in the preceding embodiments of the invention, the resonators 103 aredisposed over the stripline 102 in the position that is basically offcenter with respect to at least portions 109 and 110 of the principalconductor 105 of the stripline 102. The reduction in principal conductorwidth illustrated by portions 109 and serve the dual purpose ofproviding appropriate impedance matching to the resonators, as describedabove, as well as facilitating the off-center spacing. The resonators103 are spaced from the principal conductor 105 of the stripline 102 andfromportions 109 and 110 by dielectric spacers, the dielectric spacer106 being over portion 109, the spacer 107 being over a portion ofstripline 102 having no principal conductor within the interior of thehousing 10l,'and the spacer 108 being over the portion 110 of stripline102. The lateral distance between the resonator and the end of thestripline principle conductor is determined such that strong coupling isobtained between the stripline and the end resonator while providingnegligible coupling between the stripline and the inner resonators.These spacers may be made as explained for the preceding embodiments.

The direct-coupled band-pass filter of FIGS. 9A and 9B typically yieldsthe best results of the types we have investigated and, in addition, issmaller and simpler than the quarterwave-coupled type describedhereinafter.

The coupling characteristics between an end resonator and the striplineare set forth in FIG. 11 by curves 121 through 124, in terms of theeffective overall, Q of the filter as a function of the spacing A fromthe resonator edge to principal conductor edge, as shown in FIG. 12. Therespective curves 121 through 124 represent the differing spacerthicknesses of spacers 106 through 108 for different filter designs,specifically.

0.160 inch, 0.130 inch, 0.120 inch and 0.060 inch for all three likespacers for the respective different designs.

Except for the fact that the stripline 102 is terminated, that is opencircuited, near or beyond the end resonators 103, the coupling issimilar to that used in our band-elimination filters described above inthat the lateral offset is adjusted to the position yielding a broad,flat maximum of coupling to the resonant mode of the resonator to beutilized. Most of the. fabrication advantages of our invention and otheradvantages described above also apply here for that reason.

For the sake of completeness we list the features of the band-passfilters of FIGS. 9A and 93 as follows:

1. Strong coupling, with low Q is obtainable between the dielectricresonator and the stripline.

2. The coupling can be precisely controlled and is a function of thethickness of the dielectric spacers, for example spacers 106 through108, used to locate the resonators 103 over the stripline.

3. The coupling is relatively insensitive to the lateral off-set withrespect to the center conductor.

4. The spacing of the resonators over the center conductor reduces thedegradation of the resonator Q due to conductor loss in the bottomground plane of housing 101.

5. The clearance from the resonator to the top ground plane issufficient to limit the resonator Q degradation to an acceptable level.

6. The housing width is reduced such that spurious housing resonancesare eliminated.

7. Relatively low dielectric constant, 39, materials can be used.

A quarterwave-coupled band-pass filter is illustrated in FIGS. 10A and108. This structure resembles a number of single resonator filtersconnected in cascade and spaced from one another by odd multiples of aquarterwave length. The filter of FIGS. 10A and 108 can be compared tothat of FIGS. 9A and 9B in that the spacing between resonators 103 isabout three-fourths of a wavelength between each pair and in thatsections of the principal conductor 10S extend between the resonators sothat the only portion of stripline 102 free of principal conductor is asmall space directly under each of the resonators 103.

This configuration is larger and more difficult to fabricate than thatof FIGS. 9A and 9B.

The filter characteristics may be determined by the coupling to eachresonator.

The performance of a direct-coupled band-pass filter utilizing threeresonators is presented in FIG. 13.

It should be noted that, on the vertical axis in FIG. 13, 18 representsthe absolute value of transmission and Is"! represents the absolutevalue of return loss. The 35 dB return loss of curve 132 at fo comparesvery favorably with that of other band-pass filters. The 0.4

dB insertion loss of curve 131 at f0 corresponds to a resonator Q of3,260. The undegraded resonator Q is approximately 6,000.

A major contribution to this Q degradation is made by both the adhesiveused to bond the resonators in the circuit and the metallic tuningscrews (not shown) and can be reduced by further optimization wheredesired. No spurious transmission responses are observed in the 3.7 to4.2 GHz band. The first spurious response occurs at approximately 4.6GHzand is caused by a higher order resonator mode.

What is claimed is:

l. A microwave circuit comprising a stripline including a planardielectric substrate and a principal conductor deposited on one surfaceof said substrate, and means for filtering a portion of the frequenciesof microwave radiation'transmitted through said stripline, comprising adielectric resonator having planar surfaces parallel to the plane ofsaid substrate and having a composition selected for temperaturecompensation of its resonant frequencies, and a dielectric spacerbetween said resonator and said conductor, said resonator being offsetfrom symmetrical alignment over said conductor and adapted to support aTE 5 mode of its resonant frequencies, the degree of offset beingselected to maximize the degree of coupling of said mode from saidstripline to said resonator.

2. A microwave circuit according to claim I in which the composition ofthe dielectric resonator is selected to have positive coefficient offrequency variation with respect to temperature and the substrate of thestripline and housing effects are selected to have a negativecoefficient of frequency variation with respect to temperature. I

3. A microwave circuit according to claim 1 in which thecenter-to-center offset spacing of the dielectric resonator with respectto the principal conductor is substantially less than the half width ofthe dielectric resonator.

4. A microwave circuit according to claim 3 including a ground planehaving a metallic housing structure about the dielectric resonator, thehousing structure being selected to have the opposite coefficientfrequency variation with respect to temperature as compared to thatpossessed by the dielectric resonator.

5. A microwave circuit according to claim 4 including means for" tuningthe filter characteristic of the dielectric resonator.

6. A microwave circuit according to claim 1 including a plurality of thedielectric resonators disposed at least partially over the principalconductor and separated by odd multiples of a quarter wavelengths fromone another to minimize spurious coupling resonances therebetween, saidcircuit including a metallic housing structure intruding between saidresonators.

7. A microwave circuit according to claim 1 in which the dielectricspacer is composed of a readilymachinable dielectric material, wherebythe spacing between two of its surfaces may be accurately determined.

8. A microwave circuit according to claim 1 including a waveguidehousing that is proportioned to be beyond cut-off for a band offrequencies to be propagated and including a plurality of dielectricresonators disposed within said housing with spacings for mutualcoupling therebetween to provide propagation through the housing for theband of frequencies, the stripline comprising a planar dielectricsubstrate extending completely through the housing and a principalconductor that is broken in at least one region within said housing inproximity to one or more of the resonators.

9. A microwave circuit according to claim 8 in which the principalconductor is terminated under the end resonators of the plurality ofresonators and is absent therebetween.

10. A microwave circuit according to claim 8 in which the plurality ofdielectric resonators have spacings between adjacent resonatorssubstantially equal to a quarterwave length for the center frequency ofthe band of frequencies and the principal conductor includes segmentsextending between adjacent resonators, so that the principal conductorlacks continuity only in a plurality of limited regions respectivelyadjacent to each of the resonators.

1. A microwave circuit comprising a stripline including a planardielectric substrate and a principal conductor deposited on one surfaceof said substrate, and means for filtering a portion of the frequenciesof microwave radiation transmitted through said stripline, comprising adielectric resonator having planar surfaces parallel to the plane ofsaid substrate and having a composition selected for temperaturecompensation of its resonant frequencies, and a dielectric spacerbetween said resonator and said conductor, said resonator being offsetfrom symmetrical alignment over said conductor and adapted to support aTE01 mode of its resonant frequencies, the degree of offset beingselected to maximize the degree of coupling of said mode from saidstripline to said resonator.
 2. A microwave circuit according to claim 1in which the composition of the dielectric resonator is selected to havepositive coefficient of frequency variation with respect to temperatureand the substrate of the stripline and housing effects are selected tohave a negative coefficient of frequency variation with respect totemperature.
 3. A microwave circuit according to claim 1 in which thecenter-to-center offset spacing of the dielectric resonator with respectto the principal conductor is substantially less than the half width ofthe dielectric resonator.
 4. A microwave circuit according to claim 3including a ground plane having a metallic housing structure about thedielectric resonator, the housing structure being selected to have theopposite coefficient frequency variation with respect to temperature ascompared to that possessed by the dielectric resonator.
 5. A microwavecircuit according to claim 4 including means for tuning the filtercharacteristic of the dielectric resonator.
 6. A microwave circuitaccording to claim 1 including a plurality of the dielectric resonatorsdisposed at least partially over the principal conductor and separatedby odd multiples of a quarter wavelengths from one another to minimizespurious coupling resonances therebetween, said circuit including ametallic housing structure intruding between said resonators.
 7. Amicrowave circuit according to claim 1 in which the dielectric spacer iscomposed of a readily-machinable dielectric material, whereby thespacing between two of its surfaces may be acCurately determined.
 8. Amicrowave circuit according to claim 1 including a waveguide housingthat is proportioned to be beyond cut-off for a band of frequencies tobe propagated and including a plurality of dielectric resonatorsdisposed within said housing with spacings for mutual couplingtherebetween to provide propagation through the housing for the band offrequencies, the stripline comprising a planar dielectric substrateextending completely through the housing and a principal conductor thatis broken in at least one region within said housing in proximity to oneor more of the resonators.
 9. A microwave circuit according to claim 8in which the principal conductor is terminated under the end resonatorsof the plurality of resonators and is absent therebetween.
 10. Amicrowave circuit according to claim 8 in which the plurality ofdielectric resonators have spacings between adjacent resonatorssubstantially equal to a quarterwave length for the center frequency ofthe band of frequencies and the principal conductor includes segmentsextending between adjacent resonators, so that the principal conductorlacks continuity only in a plurality of limited regions respectivelyadjacent to each of the resonators.